The long-term objective of our laboratory is to understand the underlying molecular and genetic causes for chronic allergic airway inflammation or asthma to improve therapeutic approaches. The specific objective of this application is to dissect microRNA (miRNA)-dependent inflammatory pathways in dendritic cells (DCs) and CD4+ T cells that modulate murine allergic airway disease to fungal allergens. Asthma is a major rising global health threat that disproportionately affects combat veterans. We showed that patients with asthma often have evidence of airway mycosis, defined as detection of fungi in airway secretions in association with specific biomarkers (e.g. IgE or TH2 cells) reactive to one or more fungi. Our laboratory and others have shown that fungal proteinases and fungal spores drive asthma-like airway disease in mice. Airway epithelial cells express pattern recognition receptors (PRRs) for foreign antigens (e.g. TLRs, C-type lectin, and protease-activated receptors) and important for conventional dendritic cell (cDC) activation and initiation of allergic CD4+ type 2 cell (TH2) and TH17 cell-dependent airway inflammation, IgE production, and airway hyperresponsiveness (AHR), collectively termed allergic airway disease. We and others have shown the requirement of TH2 cells and DC in the pathogenesis of allergic airway disease. We employed murine models of chronic allergic airway inflammation, and detected aberrant expression of let-7 miRNA family in lung, CD4+ T cells and CD11c+ DCs. Specifically, the expression of let-7b and let-7c (let-7bc-cluster) is reduced in sorted lung CD4+ T cells and enhanced in CD11c+ antigen presenting cells (APCs) of mice with chronic allergic airway disease. Moreover, mice that lack the let-7bc-cluster globally or specifically in CD4+ T cells or CD11c+ APCs show blunted TH2 responses and allergic disease. On the other hand enforced expression of let-7 specifically in CD4+ T cells enhances fungus-induced allergic airway disease. The let-7b and let-7c members only account for only 9% of total let-7 activity in lymphocytes and myeloid cells suggesting that let-7bc cluster critically determines gene expression of CD4+ T cells and DCs that drive allergic sensitization to Aspergillus niger. Our central hypothesis therefore states that the let-7 family modulates allergic airway disease severity and orchestrates activation of cDCs and TH2 responses. We will address this hypothesis through the following Specific Aims: 1) Elucidate the CD4+ T cell intrinsic requirement of the let-7bc-cluster in pathogenesis of allergic airway disease. Hypothesis: The let-7bc-cluster regulates target gene expression in CD4+ T cells and controls TH2 lung inflammation and allergic airway disease. 2) Determine the role of let-7bc-cluster in DC-mediated control of allergic airway inflammation. Hypothesis: Induction of let-7bc-cluster expression in asthmatic lung cDCs directs molecular programming of TH2 driven pulmonary allergic inflammation. Through these two aims, we will further define the mechanisms by which the let-7bc cluster controls the expression of allergic airway disease, providing novel diagnostic and therapeutic insight into asthma and related disorders.